They can do this, sometimes it's a sign that you might be feeding them too small a prey size and he/she doesn't see it as worth the effort to eat it. Could also be temps not being warm enough, or enough gradient. Could also just be because it wants to. You'd need to provide a bit more information for a more educated guess at possible reasons. Enclosure size, what's in it, temps, age of snake, how long you've had it and did anything change since you last fed it, as in did you move it to bigger enclosure, the food size.